1. Field of the Invention
The present invention relates to an electrically driven injection molding machine and a power supply device for an electrically driven injection molding machine for supplying electric power to servo motors for driving the constituent parts of the electrically driven injection molding machine such as a screw, a mold opening/closing device, a molded product ejecting device.
2. Description of the Related Art
As is well known conventionally, an injection molding machine is configured by a pair of metal molds, a mold clamping device for mold-clamping these metal molds, an injection device for melting resin and injecting the resin within the metal molds etc. The injection device is configured by an injection cylinder and a screw and a plunger which are driven in the rotational direction and an axial direction within the injection cylinder etc. In the electrically driven injection molding machine, such the mold clamping device, the screw etc. are respectively driven by servo motors provided independently. In general, the injection molding process includes a measuring process for melting resin and accumulating the melted resin at the tip end portion within the cylinder, a mold clamping process for mold-clamping the metal molds, an injection process for injecting the melted resin within the metal molds thus mold-clamped, a pressure holding process for holding the pressure of the melted resin after the injection, a mold opening process for opening the metal molds, and a protruding process for protruding an ejector pin to take out a molded product from the metal molds etc. Among these processes, although the injection process requires a shorter time for the processing as compared with other processes, the injection process requires a large output power. Thus, a large servo motor of a high output is employed as the servo motor for driving the screw in the axial direction in the injection process. In recent years, needs of a so-called super-high speed injection molding has been increased which is excellent in the transfer property and can form a molded product of a complicated and fine shape and also a molded product of a thin thickness with good or excellent transfer property. An injection molding machine capable of executing such the super-high speed injection molding requires a further high output power in the injection process and so requires the servo motor of a larger scale.
As is well known conventionally, the servo motor is driven by a three-phase AC voltage generated by a servo amplifier formed by an inverter circuit. The inverter circuit is supplied with a DC voltage. The DC voltage can be obtained by rectifying the three-phase AC voltage supplied from the power receiving equipment of a factory by using a conventionally well-known converter. In the case of driving the servo motor with a high output power, a DC current supplied to the servo amplifier is required to be large. Thus, in view of the injection process in which the servo motor is required to be driven with a high output power, a three-phase AC power supply for supplying the three-phase AC voltage also requires a large current supply ability and so the power receiving equipment of a factory is required to have a relatively large power capacity. Further, since the fee structure of electric power differs depending on the maximum power to be contracted, the electricity bill becomes high when the power receiving equipment of a large power capacity is installed. In view of the requirement of the energy saving and cost reduction, a technique is demanded which can reduce the power capacity required for the power receiving equipment to thereby reduce the required electricity bill.
Japan Institute of Invention and Innovation, Journal of Technical Disclosure No. 97-6523 describes a PWM converter which is an AC/DC converter applied to the injection molding machine and including a loop for controlling the voltage of the converter and a control loop for maintaining the sine wave of the current, whereby the voltage is stabilized and the voltage can be converted into a DC current of high quality. In the AC/DC converter according to an example of the related art, since the three-phase AC current supplied from the three-phase AC power supply is controlled so as to be shaped in a sine wave, a harmonic wave causing the reduction of a power factor can be suppressed and a DC current can be obtained based on the stable voltage, advantageously. Further, since the power factor is kept to a value near 1, an amount of a reactive power is small and so the power is not consumed wastefully. Further, since a kinetic energy accumulated in the driving portion of the injection molding machine is regenerated as electric energy and returned to the power supply side as electric power, the energy efficiency is high.
However, there seem to be some points to be improved. That is, in the AC/DC converter according to the example of the related art, although the power factor can be kept to a value near 1 and an amount of a reactive power can be reduced, the maximum power required to be high in the injection process is not reduced and the electric power required in the respective processes is not smoothed. In general, the power capacity of the power receiving equipment is determined by the time average of the power consumed by production machines and working machines etc. installed in a factory, that is, an average power. When the power capacity is determined in this manner, since a large power is consumed at the time of the injection process, a phenomenon that the voltage within the factory reduces, that is, a so-called voltage drop may arise and a breaker maybe tripped. In particular, in the case where a plurality of the injection molding machines are installed in a factory, if the injection process is started by these injection molding machines simultaneously, the frequency of the occurrence of the voltage drop etc. is enhanced. Thus, it is necessary to determine the power capacity of a sufficient value in such a manner that an almost half of the maximum power necessary for the injection process is secured, and so the installation cost for the power receiving equipment becomes high.